Method of fabricating two-terminal nonlinear element using non-aqueous electrolyte

ABSTRACT

A non-aqueous electrolyte comprises an organic solvent and a solute, and also has an electrolytic conductivity that is greater than or equal to 1 mS/cm but less than or equal to 100 mS/cm. This solute preferably includes at least one of a carboxylate and a salt of inorganic oxoacid. In addition, the non-aqueous electrolyte preferably comprises water in a proportion of 1 to 10 wt %. In an MIM nonlinear element ( 20 ), an insulated film ( 24 ) is formed by anodic oxidation using the above non-aqueous electrolyte. In addition, the insulated film comprises at least one of carbon atoms and atoms of families 3 to 7 that were originally the central atoms of the salt of inorganic oxoacid, and has a relative permittivity of 10 to 25. With this MIM nonlinear element, the capacitance is sufficiently small, the steepness of the voltage-current characteristic is sufficiently large, and also the resistance is sufficiently uniform over a wide area.

TECHNICAL FIELD

This invention relates to a non-aqueous electrolyte for fabricating atwo-terminal nonlinear element that is used as a switching element, amethod of fabricating a two-terminal nonlinear element by using thisnon-aqueous electrolyte, a two-terminal nonlinear element obtained bythis fabrication method, and a liquid crystal display panel using thatelement.

BACKGROUND ART

In an active matrix type of liquid crystal display device, a liquidcrystal fills a space between an active matrix substrate, which isformed as a matrix array with a switching element provided for eachpixel region, and a facing substrate provided with a color filter or thelike. Predetermined image information can be displayed by controllingthe alignment of the liquid crystal in each pixel region. In general, athree-terminal element such as a thin-film transistor (TFT) or atwo-terminal element such as a metal-insulator-metal (MIM) type ofnonlinear element (hereinafter called a “MIM element”) is used as eachof these switching elements. A switching element using a two-terminalelement is considered to be better than a three-terminal element in thatthere is no cross-over shorting and the fabrication thereof can besimplified.

To implement a liquid crystal display panel with a high image qualitythat has good contrast, and also no discernible display unevenness,after-image, or image persistence within a liquid crystal display deviceusing MIM elements, it is important to ensure that the characteristicsof the MIM elements satisfy the following conditions:

(1) The capacitance of each MIM element must be sufficiently smallerthan the capacitance of the liquid crystal display panel,

(2) Changes with time in the voltage-current characteristic of the MIMelement must be sufficiently small,

(3) The symmetry of the voltage-current characteristic of the MIMelement must be good,

(4) The steepness of the voltage-current characteristic of the MIMelement must be sufficiently high, and

(5) The resistance of the MIM element must be sufficiently uniform overa wide area.

In other words, to increase the contrast, it is necessary to make thecapacitance of the MIM component sufficiently small with respect to thecapacitance of the liquid crystal display panel, and also ensure thatthe steepness of the voltage-current characteristic of the MIM componentis sufficiently large. To ensure there is no discernible displayunevenness, it is necessary to make the resistance of the MIM componentsufficiently uniform over a wide area. To ensure there is no discernibleafter-image, it is necessary to make sure that the changes with time inthe voltage-current characteristic of the MIM element are sufficientlysmall. Furthermore, to ensure there is no discernible image sticking, itis necessary to make sure that the changes with time in thevoltage-current characteristic of the MIM element are sufficientlysmall, and also that the symmetry of the voltage-current characteristicof the MIM element is good.

In this case, “after-image” is a phenomenon that occurs when anotherimage is displayed after a certain image has been displayed for severalminutes, in which case the previous image can still be discerned.Similarly, “image sticking” is a phenomenon that occurs when anotherimage is displayed after a certain image has been displayed for severalhours, in which case the previous image can still be discerned. Thephrase “the symmetry of the voltage-current characteristic is good”means that, when a current flows from the first conductive film to thesecond conductive film under a certain voltage and when a current flowsfrom the second conductive film to the first conductive film, thedifference in absolute values of these currents is sufficiently small.

Examples of documents that disclose techniques for MIM elements arelisted below.

(a) Japanese Patent Application Laid-Open No. 52-149090 discloses an MIMelement that is fabricated from a first conductive film of tantalum, aninsulated film that is a metal oxide film formed by anodic oxidation ofthis first conductive film, and a second conductive film of chromiumformed on a surface of this insulated film. The insulated film is formedto a uniform thickness without pinholes, by forming it by anodicoxidation of the surface of the first conductive film. Japanese PatentApplication Laid-Open No. 57-122478 disclosed the use of a diluteaqueous solution of citric acid as the electrolyte for anodic oxidation.

These techniques do not necessarily ensure sufficient quality for theabove characteristics (2) to (5) of the resultant MIM element. In otherwords, they are unsatisfactory from the viewpoints of changes with time,symmetry, and steepness of the voltage-current characteristic, and alsothe resistance of the element is not sufficiently uniform over a widearea. This means that it would be difficult to ensure a high level ofcontrast over a wide temperature range in a liquid crystal displaydevice using such MIM elements, and there will be problems such as atendency towards unevenness in the display.

(b) The international application PCT/JP94/00204 (InternationalPublication No. WO94/18600) discloses a configuration in which is used afilm of an alloy of tantalum to which tungsten is added, as the firstconductive film of the MIM element.

Since the first conductive film of the MIM element produced by thistechnique is a film of an alloy comprising tantalum and a specificelement such as tungsten, instead of tantalum alone, this provides animprovement over the techniques disclosed in the documents of (a) withrespect to characteristics (2) and (3), in other words, the changes withtime and the symmetry of the voltage-current characteristic of the MIMelement, so it is capable of improving quality to a level at whichafter-images cannot be discerned, and also of maintaining a goodcontrast over a wide temperature range. However, this technique has aproblem concerning insufficient margin in the contrast characteristicsrequired of such an element at high temperatures.

(c) Jpn. J. Appl. Phys, 31,4582 (1992) discloses the use of a diluteaqueous solution of phosphoric acid or ammonium borate as theelectrolyte for the anodic oxidation used for forming the insulated filmof an MIM element.

This technique provides an improvement over the techniques disclosed inthe documents of (a) with respect to characteristics (2) and (3), inother words, the changes with time and the symmetry of thevoltage-current characteristic of the MIM element, so it is capable ofimproving quality to a level at which after-images cannot be discerned,and also of maintaining a good contrast over a wide temperature range.However, this also has problems in that the reliability of the resultantelements is low, they are likely to be destroyed by short-circuiting,and display unevenness easily occurs.

(d) Japanese Patent Application Laid-Open No. 2-93433 discloses aconfiguration in which a film of an alloy of tantalum and silicon isused as the first conductive film of the MIM element.

This technique made it possible to improve the steepness of thevoltage-current characteristic in comparison with the techniques of thedocuments of (a), and also provide sufficient margin over a widetemperature range to ensure a high contrast. However, this techniquealso has problems in that the reliability of the element is low and thusit can easily be destroyed, and display unevenness can easily occur.

DISCLOSURE OF THE INVENTION

An objective of this invention is to provide a two-terminal nonlinearelement that satisfies all the characteristics (1) to (5) required ofthe above described MIM element, particularly a capacitance that issufficiently small, a voltage-current characteristic with a sufficientlylarge steepness, and a resistance that is sufficiently uniform over awide area; and also a liquid crystal display panel that uses thistwo-terminal nonlinear element and has a high image quality with a goodcontrast and no display unevenness.

Another objective of this invention is to provide a non-aqueouselectrolyte for fabricating a two-terminal nonlinear element having theabove described superior characteristics, as well as a fabricationmethod using that electrolyte.

As used herein, the term “non-aqueous” refers to the primary characterof the electrolyte, meaning that the electrolyte is not an aqueouselectrolyte comprising a substantial portion of water. That is, the“non-aqueous” electrolyte comprises an organic solvent and solute. Theelectrolyte can also include water, in an amount of from 1 to 10 wt %,while still being a “non-aqueous” electrolyte.

The non-aqueous electrolyte for fabricating a two-terminal nonlinearelement (hereinafter called a “MIM nonlinear element”) in accordancewith a first aspect of this invention comprises an organic solvent andsolute, and also has an electrolytic conductivity that is greater thanor equal to 1 mS/cm but less than or equal to 100 mS/cm, but ispreferably greater than or equal to 1 mS/cm but less than or equal to 10mS/cm.

Using this electrolyte to perform anodic oxidation on the firstconductive film, which is formed on the substrate of tantalum or atantalum alloy, makes it possible to form an oxide film which has auniform film quality and which also has a thickness that is sufficientlyuniform over the entire surface of the substrate. Therefore, an MIMnonlinear element obtained by anodic oxidation using this electrolytehas a resistance that is sufficiently uniform over a wide area. In anMIM nonlinear element in accordance with this invention, the material ofthe second conductive film is not limited to a metal; the definitionthereof also comprises a conductive film of a material such as indiumtin oxide (ITO).

The permeation of the solute or solvent, or both solute and solvent,into the oxide film during the anodic oxidation makes it possible toreduce the relative permittivity of the oxide film (insulated film) towithin a suitable range. As a result, the thus obtained MIM nonlinearelement has a capacitance that is sufficiently small, and also thesteepness of the voltage-current characteristic thereof is high.

The solute may comprise at least one of a carboxylate and a salt ofinorganic oxoacid.

This carboxylate may be at least one salt of carboxylic acids selectedfrom the group of aromatic carboxylic acids and aliphatic dicarboxylicacids having no hydroxyl groups. This carboxylate is preferably anaromatic carboxylate, and at least one of salicylate and phthalate isparticularly preferable.

The central atom of the oxoacid in the salt of inorganic oxoacid may bean atom belonging to one of Groups IIIA, IIIB, IVA, IVB, VA, VB, VIA,VIB, VIIA, and VIIB (CAS version) of the periodic table. In addition,this salt of inorganic oxoacid may be at least one type selected fromthe group of nitrates, vanadates, phosphates, chromates, tungstates,molybdates, silicates, perrhenates, borates, and sulfates. A tungstateis preferable as this salt of inorganic oxoacid, and at least one typeof primary, secondary, tertiary, and quaternary ammonium salt isparticularly preferable.

The non-aqueous electrolyte of this invention comprises an organicsolvent and a solute, and it may also comprise water where theproportion thereof with respect to the electrolyte is preferably 1 to 10wt %.

This organic solvent may be at least one of ethylene glycol andγ-butyrolactone.

An MIM nonlinear element in accordance with a second aspect of thisinvention comprises a first conductive film, insulated film, and secondconductive film deposited in sequence on a substrate, wherein the firstconductive film is of tantalum or a tantalum alloy; and wherein theinsulated film is formed by anodic oxidation of the first conductivefilm, comprises carbon atoms or at least one type of element belongingto at least one of Groups IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, VIIA,and VIIB of the periodic table and originating from the central atom ofan inorganic oxoacid, and also has a relative permittivity of 10 to 25.The relative permittivity of the insulated film may be more preferably22 to 25, to ensure sufficiently small changes with time in the MIMnonlinear element.

The carbon atoms or at least one type of element belonging to at leastone of families 3 to 7 of the periodic table and originating from thecentral atom of an inorganic oxoacid, comprised within the insulatedfilm, may be distributed through the entire thickness direction of theinsulated film.

The relative intensity of these carbon atoms with respect to oxygenatoms (¹⁸O) may be 0.2 to 1000 throughout the entire thickness directionof the insulated film, and more preferably 0.2 to 100, as determined byelemental analysis obtained by secondary ion mass spectrometry (SIMS) byirradiation of cesium primary ions.

The relative intensity of this at least one element in the insulatedfilm belonging to Groups IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, VIIA,and VIIB; of the periodic table may be at least 10 times the intensityof the element in the first conductive film, as determined by elementalanalysis obtained by SIMS.

The insulated film of the MIM nonlinear element in accordance with thisinvention is obtained by anodic oxidation of the first conductive filmin the non-aqueous electrolyte of this invention. The relativepermittivity of the insulated film can be reduced to a suitable range bythe permeation of the solute or solvent, or both the solute and solvent,into the oxide film during the anodic oxidation, which causes at leastcarbon atoms or at least one element belonging to Groups IIIA, IIIB,IVA, IVB, VA, VB, VIA, VIB, VIIA, and VIIB of the periodic table andoriginally the central atoms of an inorganic oxoacid to becomeincorporated into the oxide film (insulated film). As a result, the MIMnonlinear element of this invention has superior characteristics such asa low capacitance and a large steepness of the voltage-currentcharacteristic thereof.

A liquid crystal display panel in accordance with a third aspect of thisinvention includes the above described MIM nonlinear element. Morespecifically, it comprises a first substrate provided with a transparentsubstrate, one type of signal line disposed in a predetermined patternon the substrate, a MIM nonlinear element in accordance with thisinvention, connected at a predetermined pitch to this signal line, and apixel electrode connected to the MIM nonlinear element; a secondsubstrate provided with another type of signal line positioned oppositeto the pixel electrode; and a liquid crystal layer interposed betweenthe first substrate and the second substrate. This liquid crystaldisplay panel has a good contrast, is not likely to develop problemssuch as display unevenness, and therefore makes it possible to provide ahigh-quality image display such that it can be applied to a wide rangeof applications.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of an essential component of a liquid crystaldisplay panel in which is used an MIM nonlinear element of thisinvention.

FIG. 2 is a cross-sectional view taken along the line II—II of FIG. 1.

FIG. 3 is a cross-sectional view through another example of an MIMnonlinear element of this invention.

FIG. 4 shows an equivalent circuit of the liquid crystal display panelof this invention.

FIG. 5 is a perspective view of a liquid crystal display panel inaccordance with this invention.

FIG. 6 is a plan view of essential components of a liquid crystaldisplay panel to which another MIM nonlinear element in accordance withthis invention is applied.

FIG. 7 is a cross-sectional view taken along the line VII—VII in FIG. 6.

FIG. 8 is a graph of the relationship between applied voltage andcurrent for MIM nonlinear elements in accordance with Example 1 of thisinvention and Comparative Example 4.

FIG. 9 shows the profiles of carbon atoms and oxygen atoms in theinsulated film and first conductive film of an MIM nonlinear element inaccordance with Example 1 of this invention, obtained by SIMS.

FIG. 10 shows the profiles of carbon atoms and oxygen atoms in theinsulated film and first conductive film of an MIM nonlinear element ofComparative Example 4, obtained by SIMS.

FIG. 11 shows the profiles of elements in the insulated film and firstconductive film of an MIM nonlinear element in accordance with Example12 of this invention, obtained by SIMS.

FIG. 12 shows the profiles of elements in the insulated film and firstconductive film of an MIM nonlinear element of Comparative Example 6,obtained by SIMS.

FIG. 13 shows a SIMS spectrum obtained for an MIM nonlinear element inaccordance with Example 30 of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiment of this invention is described below with referenceto the accompanying drawings.

MIM Nonlinear Element and Liquid Crystal Display Panel

A schematic plan view of a single liquid crystal drive electrode usingthe MIM nonlinear element of this invention is shown in FIG. 1, with aschematic cross-sectional view thereof taken along the line II—II inFIG. 1 being shown in FIG. 2.

This MIM nonlinear element 20 is configured of a substrate (firstsubstrate) 30 formed of a material having insulating properties as wellas transparency, such as glass or plastic; an insulated film 31 formedon a surface of the substrate 30; a first conductive film 22 formed oftantalum or a tantalum alloy; an insulated film 24 formed by anodicoxidation of a surface of the first conductive film 22; and a secondconductive film 26 formed on a surface of the insulated film 24. Thefirst conductive film 22 of this MIM nonlinear element 20 is connectedto a signal line (scan line or data line) 12 and the second conductivefilm 26 is connected to a pixel electrode 34.

The insulated film 31 is formed of a material such as tantalum oxide.This insulated film 31 is formed with the objectives of ensuring thatthere is no peeling of the first conductive film 22 due to heattreatment after the second conductive film 26 has been superimposedthereon, and also to prevent dispersion of impurities from the substrate30 to the first conductive film 22. If these problems are not likely tooccur, however, this film is not necessary.

The first conductive film 22 could be of tantalum alone, or it could bean alloy film having tantalum as the main component and comprising anelement from groups 6, 7, and 8 of the periodic table. Preferableexamples of the element added to this alloy could be tungsten, chromium,molybdenum, rhenium, yttrium, lanthanum, or dysprosium. Tungsten isparticularly preferable as this added element, with the proportionthereof being 0.2 to 6 atom %, for example.

The insulated film 24 is formed by anodic oxidation in a specificnon-aqueous electrolyte, as will be described later. This insulated film24 comprises the added element comprised with the first conductive film22 and also at least one substance taken from the organic solvent andsolute within the electrolyte, preferably comprising carbon atoms and/orthe central atom of salt of an inorganic oxoacid, or hydrogen. By addingcarbon atoms or the central atom of the salt of inorganic oxoacid to theinsulated film 24 in this manner, it is possible to make the relativepermittivity of the insulated film 24 less than that of ordinarytantalum oxide. This relative permittivity is set to be 10 to 25,preferably 22 to 25. As a result, the capacitance of the MIM nonlinearelement 20 can be made sufficiently small and also the steepness of thevoltage-current characteristic thereof can be make sufficiently large.

The carbon atoms or the central atom of the salt of inorganic oxoacidcomprised within the insulated film 24 are preferably distributedthroughout the thickness direction of this insulated film 24, in otherwords, throughout the entire region from the boundary between theinsulated film 24 and the first conductive film 22 to the boundarybetween the insulated film 24 and the second conductive film 26. Bydistributing the carbon atoms or the central atom of the salt ofinorganic oxoacid throughout the insulated film 24 in this manner, it ispossible to cause a change in the capacitance of the insulated film andalso reduce the relative permittivity of the insulated film.

The profile of these carbon atoms can be confirmed by a method such assecondary ion mass spectrometry (SIMS) by cesium ion etching. With thisanalysis method, the relative intensity thereof with respect to oxygenatoms (¹⁸O) is preferably 0.2 to 1000 throughout the entire thicknessdirection of this insulated film 24; more preferably 0.2 to 100.

The profile of the element belonging to any of groups 3 to 7, whichoriginates from this central atom of the salt of inorganic oxoacid, canbe confirmed by SIMS by irradiation of cesium primary ions or the like.With this analysis method, the intensity of this element belonging togroups 3 to 7 of the periodic table in the insulated film is at least 10times the intensity thereof in the first conductive film; morepreferably 10 to 10000 times that intensity. The intensity within thisfirst conductive film can be specified as an average value of intensity(as, for example, the number of secondary ions) within a region of thefirst conductive film where the profile of that particular element issubstantially constant.

Note that the boundary between the insulated film and the firstconductive film is defined in this analysis method as a surfacecorresponding to the intermediate point between the intensity of ¹⁸O inthe insulated film and the intensity of ¹⁸O in the first conductivefilm, from the ¹⁸O profile obtained by SIMS, for example.

The second conductive film 26 is not particularly limited, but it isusually formed of chromium. Similarly, the pixel electrode 34 is formedas a transparent conductive film of a material such as indium tin oxide(ITO).

In addition, the second conductive film and the pixel electrode can beformed from the same transparent conductive film 36, as shown in FIG. 3.This use of the same film for both the second conductive film and thepixel electrode makes it possible to reduce the number of fabricationsteps required for film formation.

The description now turns to an example of a liquid crystal displaypanel in which this MIM nonlinear element 20 is used.

A typical equivalent circuit of an active matrix type of liquid crystaldisplay panel using this MIM nonlinear element 20 is shown in FIG. 4.This liquid crystal display panel 10 comprises a scan signal drivecircuit 100 and a data signal drive circuit 110. The liquid crystaldisplay panel 10 is provided with signal lines, in other words, aplurality of scan lines 12 and a plurality of data lines 14, where thescan lines 12 are driven by the scan signal drive circuit 100 and thedata lines 14 are driven by the data signal drive circuit 110. At eachof a plurality of pixel regions 16, the MIM nonlinear element 20 and aliquid crystal display element (liquid crystal layer) 41 are connectedin series between a scan line 12 and a data line 14. Note that in FIG. 4the MIM nonlinear element 20 is shown connected to the scan line 12 sideand the liquid crystal display element 41 is shown connected to the dataline 14 side, but the opposite configuration in which the MIM nonlinearelement 20 is connected to the data line 14 side and the liquid crystaldisplay element 41 is connected to the scan line 12 side is alsopossible.

A schematic perspective view of an example of the configuration of aliquid crystal display panel in accordance with this embodiment is shownin FIG. 5. This liquid crystal display panel 10 is provided with twoopposing substrates, in other words, a first substrate 30 and a secondsubstrate 32, with a liquid crystal filling the space between thesubstrates 30 and 32. The insulated film 31 is formed on the firstsubstrate 30, as previously described. The plurality of signal lines(scan lines) 12 are provided on the surface of this insulated film 31.The plurality of data lines 14 are formed in strips on the secondsubstrate 32 in such a manner as to cross the scan lines 12. Each pixelelectrode 34 is connected to a scan line 12 by the corresponding MIMelement 20.

The display behavior of the liquid crystal display element 41 iscontrolled by switching it to a display state, a non-display state, oran intermediate state, based on signals applied to the scan line 12 anddata line 14. Any generally-used method can be used for controlling thisdisplay behavior.

Another embodiment of the MIM nonlinear element is shown in FIGS. 6 and7. FIG. 6 is a schematic plan view of a single liquid crystal driveelectrode using the MIM nonlinear element of this embodiment, and FIG. 7is a schematic cross-sectional view taken along the line VII—VII of FIG.6.

This MIM nonlinear element 40 differs from the previously described MIMnonlinear element 20 in having a back-to-back structure. In other words,the MIM nonlinear element 40 has a configuration in which a first MIMnonlinear element 40 a and a second MIM nonlinear element 40 b areconnected in series but with opposite polarities.

More specifically, the MIM nonlinear element 40 is configured of asubstrate (first substrate) 30 formed of a material having insulatingproperties as well as transparency, such as glass or plastic; aninsulated film 31 formed on a surface of the substrate 30; a firstconductive film 42 formed of tantalum; an insulated film 44 formed byanodic oxidation of a surface of the first conductive film 42; and twosecond conductive films 46 a and 46 b formed on a surface of theinsulated film 44, but mutually isolated from one another. The secondconductive film 46 a of the first MIM nonlinear element 40 a isconnected to a signal line (either a scan line or a data line) 48 andthe second conductive film 46 b of the second MIM nonlinear element 40 bis connected to a pixel electrode 45. The thickness of the insulatedfilm 44 is set to be thinner than that of the insulated film 24 of theMIM nonlinear element 20 shown in FIGS. 1 and 2, such as, for example,approximately half the thickness thereof.

Electrolyte

The description now turns to details of the electrolyte used in theanodic oxidation of this invention.

The electrolyte of this invention is a non-aqueous electrolytecomprising an organic solvent and a solute. The electrolyticconductivity of the non-aqueous electrolyte is 1 to 100 mS/cm,preferably 1 to 10 mS/cm. In general, the electrolytic conductivity ofthe electrolyte depends on the concentration of the solute, such that ahigher solute concentration gives a higher electrolytic conductivity, soit is preferable that the solute dissolves easily in the organicsolvent.

This solute is not particularly limited and can be any salt of anorganic acid or inorganic acid, but a carboxylate or a salt of inorganicoxoacid is preferable.

An example of this carboxylate could be at least one salt of carboxylicacid selected from aromatic carboxylic acids and aliphatic dicarboxylicacids having no hydroxyl groups, for example. The carboxylic acid ispreferably one with a low molecular weight, having 2 to 8 carbon atoms.

Specific examples of aromatic carboxylic acids include: aromaticmonocarboxylic acids such as benzoic acid, toluic acid, salicylic acid,and resorcylic acid; and aromatic dicarboxylic acids such as phthalicacid. Specific examples of aliphatic dicarboxylic acid having nohydroxyl groups include saturated carboxylic acids such as oxalic acid,malonic acid, succinic acid, glutaric acid, and adipic acid; andunsaturated carboxylic acids such as maleic acid and citraconic acid.

The salt of inorganic oxoacid is preferably a salt such that the centralatom of the oxoacid is an atom belonging to any of Groups IIIA, IIIB,IVA, IVB, VA, VB, VIA, VIB, VIIA, and VIIB of the periodic table. Theoxoacid could be one in which the central atom is not a metal, such asnitric acid, phosphoric acid, boric acid, silicic acid, or sulfuricacid; or it could be one in which the central atom is a metal, such aschromic acid, vanadic acid, tungstic acid, molybdic acid, or perrhenicacid. Furthermore, the oxoacid could be a polyacid. It may be even aniso-polyacid or a hetero-polyacid.

The cations forming the salt could be ammonium ions; alkaline metalions; primary, secondary, tertiary, or quaternary alkylammonium ions;phosphonium ions; or sulfonium ions; but ammonium ions and primary,secondary, tertiary, or quaternary alkylammonium ions are particularlypreferable. With alkylammonium ions, it is best to select the size ofthe alkyl group from consideration of solubility in the organic solvent.

The salt of organic acid as the solute is preferably ammoniumsalicylate, ammonium γ-resorcylate, ammonium benzoate, ammonium hydrogenphthalate, diammonium phthalate, ammonium malonate, ammonium adipate, orammonium maleate; but ammonium salicylate, ammonium hydrogen phthalate,or diammonium phthalate is most preferable.

The salt of inorganic acid as the solute is preferably a tungstate. Thistungstate could be an ortho-acid salt comprising a WO₄ ²⁻ group, apara-acid salt comprising a WO₆ ⁶⁻ group, or a polyacid salt which is adehydration-condensate of the ortho-acid salt or para-acid salt.

The cation component of this tungstate is preferably an inorganic ionsuch as lithium, sodium, potassium, or ammonium; or an organic ionhaving an organic group such as one of primary, secondary, tertiary, orquaternary ammonium, phosphonium, or sulfonium. From considerations ofsolubility in the solvent, an organic ion is preferable, and fromconsiderations of economy, a primary, secondary, tertiary, or quaternaryammonium compound is particularly preferable. From considerations ofelectrolytic conductivity, this primary, secondary, tertiary, orquaternary ammonium compound is preferably one in which the organicsubstituent is a hydrocarbon group with 1 to 4 carbon atoms; morepreferably it is a heterocyclic ammonium compound in which alkyl groupsare linked together.

Specific examples of such ammonium salts include: an aliphatic primaryammonium compound such as methylammonium or ethylammonium; an aliphaticsecondary ammonium compound such as dimethylammonium or diethylammonium;an aliphatic tertiary ammonium compound such as trimethylammonium,dimethylethylammonium, methyldiethylammonium, or triethylammonium; analiphatic heterocyclic tertiary ammonium compound such as1-methylimidazolinium, 1,2-dimethylimidazolinium,1-ethyl-2-methylimidazolinium, 1,2,4-trimethylimidazolinium,1-methyl-1,4,5,6-tetrahydropyrimidinium,1,2-dimethyl-1,4,5,6-tetrahydropyrimidinium, or 5-azonia-1,5-diazabicyclo[4.3.0]nonene-5,8-azonia-1,8-diazabicyclo[5.4.0]undecene-7; an aromatic heherocyclictertiary ammonium compound such as pyridinium, 1-methylimidazolium,1-ethylimidazolium, or 1,2-dimethylimidazolium; an aliphatic quaternaryammonium compound such as tetramethylammonium, timethylethylammonium,dimethylidiethylammonium, methyltriethylammonium, or tetraethylammonium;an aliphatic heterocyclic quaternary ammonium compound such as1,1-dimethylpyrrolidinium, 1-methyl-1-ethylphrrolidinium,1,1-diethylpyrrolidinium, 1,1-dimethylpiperidinium,1-methyl-1-ethylpiperidinium, 1,1-diethylpiperidinium,1,1-tetramethylenepyrrolidinium, 1,1-pentamethylenepyrrolidinium,1,1-tetramethylenpiperidinium, 1,1-pentamethylenepiperidinium,1,2,3-trimethylimidazolinium, 1,2,3,4-tetramethylimidazolinium, or1-ethyl-2,3-dimethylimidazolinium; or an aromatic heterocyclicquaternary ammonium compound such as 1-ethylpyridinium,1-butylpyridinium, 1,3-dimethylimidazolium, 1,2,3-trimethylimidazolium,1-ethyl-3-methylimidazolium, or 1-ethyl-2,3-dimethylimidazolium.

Among these, salts of triethylmethylammonium and tetraethylammonium areparticularly preferable from considerations of solubility andelectrolytic conductivity.

The organic solvent could be at least one selected from the group of analcohol-related solvent such as ethylene glycol or methyl cellosolve; alactone-related solvent such as γ-butyrolactone, γ-valerolactone orδ-valerolactone; a carbonate-related solvent such as ethylene carbonate,propylene carbonate, or butylene carbonate; an amide-related solventsuch as N-methylformamide, N-ethylformamide, N,N-dimethylformamide,N,N-diethylformamide, N-methylacetamide, N,N-dimethylacetamide, orN-methylpyrrolidinone; a nitrile-related solvent such as3-methoxypropionitrile or glutaronitrile; a phosphate-related solventsuch as trimethylphosphate or triethylphosphate; or a mixture thereof. Anon-polar solvent such as hexane, toluene, or silicone oil could beadded to any of these organic solvents.

Of the above described organic solvents, ethylene glycol andγ-butyrolactone are preferable, either alone or mixed.

The concentration of the solute is determined from considerations of theelectrolytic conductivity of the electrolyte, and the quantity of soluteto be incorporated into the anodic oxidation film, and pH of theelectrolyte. The concentration of the carboxylate dissolved in theorganic solvent, for example, is preferably 1 to 30 wt %; morepreferably 1 to 10 wt %. The concentration of the salt of inorganicoxoacid dissolved in the organic solvent is preferably 1 to 30 wt %;more preferably 1 to 10 wt %.

It is further preferable that water is comprised within the electrolyte,in addition to the organic solvent and solute. The proportion of thiswater is preferably 1 to 10 wt %. If the solute is an aromaticcarboxylate, the proportion of this water is preferably 3 to 10 wt %;more preferably 5 to 7 wt %. If the solute is a tungstate, theproportion of this water is preferably 1 to 10 wt %; more preferably 3to 5 wt %.

If an aromatic carboxylate is used as the solute, the pH of thenon-aqueous electrolyte as measured by a pH-meter is preferably 4 to 12,more preferably 4.5 to 10, and even more preferably 5 to 8. If the pH ofthe non-aqueous electrolyte is greater than 12, the oxide film will belikely to peel off; if the pH is less than 4, the electrolyticconductivity of the electrolyte will be too low, making it difficult toform a uniform oxide film. The salt of inorganic acid as the solute ismost preferably a tungstate.

If the non-aqueous electrolyte comprises a tungstate, the pH thereof asmeasured by a pH-meter is preferably 8 to 13; more preferably 9 to 12.If the pH is greater than 13, the oxide film will be likely to peel off;if the pH is less than 8, the electrolytic conductivity of theelectrolyte will be too low, making it difficult to form a uniform oxidefilm.

This invention makes it possible to increase the steepness of thevoltage-current characteristic and also reduce the changes with time ofthe voltage-current characteristic, by the permeation of substancescomprised within the solute, solvent, or both, such as tungsten atomsfrom within the solute and carbon atoms from within the solvent, intothe oxide film during the anodic oxidation thereof.

By dissolving a substance such as a tungstate as the solute, it ispossible to ensure that tungsten permeates at a suitable lowconcentration, more specifically at a concentration within the range of0.001 to 0.5 atom %, and also uniformly into the oxide film obtained bythe anodic oxidation. The reasons why this improves the steepness of thevoltage-current characteristic and changes with time of thevoltage-current characteristic are not absolutely clear, but it isthought to be because the tungsten alleviates the effects of excessoxygen in an anodic oxide film usually comprising more oxygen thanstoichiometric composition of tantalum oxide (Ta₂O₅). It has beenconfirmed that the use of an organic solvent in the electrolyte ensuresthat the capacitance of the MIM nonlinear element is sufficiently small.

Fabrication of MIM Nonlinear Element

The description now turns to the method of fabricating the MIM nonlinearelement 20 of FIG. 2. The fabrication method of this invention ischaracterized in that the first conductive film is subjected to anodicoxidation in a specific non-aqueous electrolyte.

This MIM nonlinear element 20 is fabricated by the process describedbelow by way of example.

(a) First of all, the insulated film 31 is formed of tantalum oxide onthe substrate 30. The insulated film 31 could be formed by, for example,thermal oxidation of a tantalum film deposited by sputtering, or bysputtering or co-sputtering using a target of tantalum oxide. Thisinsulated film 31 is provided to improve the adhesiveness of the firstconductive film 22 and also prevent diffusion of impurities from thesubstrate 30, so it is formed to a thickness of, for example,approximately 50 to 200 nm.

The first conductive film 22 is then formed of tantalum or a tantalumalloy on the insulated film 31. The thickness of the first conductivefilm is selected to be suitable for the utility of the MIM nonlinearelement, and would ordinarily be on the order of 100 to 500 nm. Thisfirst conductive film could be formed by sputtering or electron beamdeposition. As a method of forming the first conductive film from atantalum alloy, sputtering with a mixed target, co-sputtering, electronbeam deposition, or a similar method could be used. The other elementcomprised within the tantalum alloy could be selected from thepreviously described elements from groups 6, 7, and 8 of the periodictable, preferably tungsten, chromium, molybdenum, rhenium, or the like.

The first conductive film 22 is patterned by a generally-usedphotolithography and etching technique. The signal lines (scan lines ordata lines) 12 are formed by the same fabrication step as that of thefirst conductive film 22.

(b) The surface of this first conductive film 22 is then oxidized byanodic oxidation to form the insulated film 24. During this process, thesurfaces of the signal lines 12 are also oxidized to form the insulatedfilm thereon. The thickness of this insulated film 24 is preferablyselected from considerations of the utility thereof, and is, forexample, on the order of 20 to 70 nm.

The anodic oxidation is performed within a temperature range at whichthe electrolyte is a stable liquid. This temperature range is generally−20° C. to 150° C., but is preferably room temperature to 100° C. Themethod used to control the current and voltage during the anodicoxidation is not specifically limited, but it is usual to perform anodicoxidation at a constant current up to a previously determinedelectrolytic voltage (Vf) then, after the electrolytic voltage Vf hasbeen reached, maintain that voltage for a fixed time. The currentdensity during this time is preferably 0.001 to 10 mA/cm²; morepreferably 0.01 to 1 mA/cm². The electrolytic voltage Vf during thefabrication of the liquid crystal display panel also depends on thedesign of the drive circuit but it is usually 5 to 100 V; preferably 10to 40 V.

The electrolyte used in the anodic oxidation is a non-aqueouselectrolyte having a specific electrolytic conductivity, as describedpreviously, and this electrolyte makes it possible to obtain aninsulated film having uniform film thickness and characteristics withinthe substrate surface. During the anodic oxidation, at least one of thesolute and solvent within the electrolyte permeates into the oxide film,such that the relative permittivity of the insulated film can be set towithin a suitable range by the permeation into the oxide film of anelement within the solute or solvent, preferably carbon and/or thecentral atom of the salt of inorganic oxoacid, or hydrogen.

(c) A film of a metal such as chromium, aluminum, titanium, ormolybdenum is then deposited by a method such as sputtering, to form thesecond conductive film 26. The second conductive film is formed to athickness of 50 to 300 nm, for example, and is subsequently patternedwith generally-used photolithography and etching techniques. An ITO filmis then deposited thereon to a thickness of 30 to 200 nm by a methodsuch as sputtering, then the pixel electrode 34 is formed to apredetermined pattern by generally-used photolithography and etchingtechniques.

Note that the second conductive film and the pixel electrode in the MIMnonlinear element 20 shown in FIG. 3 are formed from the sametransparent conductive film 36, which is an ITO film or the like. Sincethe second conductive film and the pixel electrode are formed in thesame step in this case, it is possible to make the fabrication processsimpler.

EXAMPLES

Specific examples of this invention are described in more detail below,together with.

Example 1

A 200-nm-thick tantalum film was deposited by sputtering on a glasssubstrate, to form a first conductive film. Using an ethylene glycolsolution comprising 10 wt % of ammonium salicylate as the electrolyte,electrolysis at constant current was then performed at a current densityof 0.1 mA/cm² until the voltage reached 30 V, followed by electrolysisat constant voltage at 30 V for approximately two hours, to subject thetantalum film to anodic oxidation. As a result, a tantalum oxide film ofa thickness of approximately 50 nm was formed. Note that when aconductometer was used to measure the electric conductivity of theelectrolyte it was found to be 2.74 mS/cm.

Heat treatment at 400° C. was then performed in an atmosphere ofnitrogen, to stabilize the anodic oxide film (insulated film), thenchromium was deposited by sputtering to a thickness of 150 nm on thatinsulated film to form the second conductive film, completing the MIMnonlinear element. The characteristics of the MIM nonlinear element weremeasured, as described below.

Relative Permittivity

The electrostatic capacity of an MIM nonlinear element having a 4-μmsquare pattern, fabricated by the above described method, was found tobe 51 fF. Note that this electrostatic capacity was measured byconnecting together 1000 of these 4-μm-square MIM nonlinear elements inparallel and applying an AC voltage with a frequency of 10 kHz thereto.The thickness of the insulated film was measured with an ellipsometerand found to be 48.3 nm. The relative permittivity of the insulated filmwas calculated to be 17.5, from the thus obtained electrostatic capacityand film thickness.

β Value

The voltage-current characteristic of this MIM nonlinear element wasalso measured, and the average values thereof were plotted to give thegraph shown in FIG. 8. The curve denoted by reference number 801 in FIG.8 is data for Example 1, and a nonlinear coefficient (β value)expressing the steepness of this curve was derived from the slopethereof and found to be 6.1.

r Value

To investigate the uniformity of elements, a voltage of 10 V was appliedto five MIM nonlinear elements fabricated on the same substrate. Thedifference between the maximum and minimum values of logarithms of thecurrents (units: amperes) flowing through these elements was derived andfound to be 0.24 (hereinafter, this value is called “r value”). Notethat the following three levels are used in evaluating this r value:

⊚ 0.5 or less

◯ 0.5 to 1.0

x 1.0 or greater

The electrolytic conductivity of the electrolyte and the relativepermittivity of the insulated film, β value, and r value are listed inTable 1.

When a liquid crystal display panel was fabricated by using MIMnonlinear elements formed by the above described method, a contrast ofat least 100 was obtained over a temperature range of 0° C. to 80° C.,and no display unevenness was discerned.

Comparative Example 1

An electrolyte of ethylene glycol with 2 wt % citric acid was usedinstead of the electrolyte of Example 1; otherwise, MIM nonlinearelements were fabricated in a similar manner to that of Example 1. Theelectrolytic conductivity of the electrolyte, the relative permittivityof the insulated film, β value, and r value of these MIM nonlinearelements were obtained in the same manner as that of Example 1. Theresults are listed in Table 1.

Examples 2 to 6 and Comparative Examples 2 and 3

The electrolytes listed below were used instead of the electrolyte ofExample 1; otherwise, MIM nonlinear elements were fabricated in asimilar manner to that of Example 1. In other words, an ethylene glycolsolution comprising 9 wt % of diammonium maleate was used as theelectrolyte for Example 2, an ethylene glycol solution comprising 10 wt% of diammonium malonate was used as the electrolyte for Example 3, aγ-butyrolactone solution comprising 10 wt % ammonium salicylate was usedas the electrolyte for Example 4, an ethylene glycol solution comprising10 wt % of ammonium nitrate was used as the electrolyte for Example 5,an ethylene glycol solution comprising 10 wt % of tetraethyl-ammoniumvanadate was used as the electrolyte for Example 6, and an ethyleneglycol solution comprising 2 wt % of diammonium tartrate was used as theelectrolyte for Comparative Example 2. Water was used as the solvent forComparative Example 3.

The electrolytic conductivity of the electrolyte and the relativepermittivity of the insulated film, β value, and r value were obtainedfor each of the thus obtained MIM nonlinear elements, in a similarmanner to Example 1. The results are listed in Table 1.

TABLE 1 Electrical Conducti- Relative Electrolyte vity Permitti- β rSolute Solvent (mS/cm) vity Value Value E 1 Ammonium EG 2.74 17.5 6.1 ⊚salicylate E 2 Diammonium EG 3.36 15.6 6.0 ⊚ maleate E 3 Diammonium EG3.68 20.6 5.5 ⊚ malonate E 4 Ammonium GBL 1.34 21.7 3.8 ∘ salicylate E 5Ammonium EG 8.55 20.7 5.6 ∘ nitrate E 6 Tetraeth- EG 1.0 20.6 5.7 ∘yl-ammoni- um vana- date CE Citric EG 0.05 14.0 6.0 x 1 acid CEDiammonium EG 0.05 14.0 6.0 x 2 tartrate CE Citric Water 10 27.0 4.0 ⊚ 3acid E: Example; CE: Comparative Example; EG: ethylene glycol; GBL:γ-butyrolactone

As is clear from Table 1, it was determined that the relativepermittivity of the insulated film was small and the β value whichindicates the steepness of the voltage-current characteristic wassufficiently large, in each of these examples of the invention. It wasalso determined that, since the electrolytic conductivity of theelectrolyte was sufficiently large in each of these examples of theinvention, the r value thereof was small, in other words, a homogeneousoxide film can be obtained thereby.

In contrast thereto, the electrolytic conductivity of the electrolytesof Comparative Examples 1 and 2 were below the range of these examples.With Comparative Examples 1 and 2, it is true that the relativepermittivity of the insulated film was small and the β values weresufficiently large, but there was a large dispersion in thevoltage-current characteristics and both examples could only producenon-homogeneous oxide films wherein a r value was too large to bemeasured. When the usual drive voltage was applied to the elementsobtained by Comparative Examples 1 and 2, some of the elementsshort-circuited and were destroyed. Comparative Example 3 had a worse βvalue than these examples of the invention.

The description now turns to separate experimental examples relating tothe MIM nonlinear element of Example 1.

SIMS

SIMS by cesium ion etching was first performed to obtain the profile ofcarbon atoms comprised within this insulated film, with the resultsbeing as shown in FIG. 9. Thickness from the insulated film surfacetowards the first conductive film is plotted along the horizontal axisin FIG. 9 and numbers of atoms (logarithmic) are plotted along thevertical axis. Reference number 901 denotes the profile of oxygen atoms(¹⁸O), reference number 902 denotes the profile of carbon atoms,reference symbol D1 denotes the region of the insulated film, andreference symbol D2 denotes the partial region of the first conductivefilm.

It was determined from FIG. 9 that carbon atoms are distributeduniformly in the thickness depth direction through the insulated film.The quantity of carbon atoms was determined to be 0.2 to 100, in termsof the relative intensity thereof with respect to oxygen atoms (¹⁸O).

Comparative Example 4

An aqueous solution of 0.1 wt % of citric acid was used instead of theelectrolyte of Example 1; otherwise, MIM nonlinear elements werefabricated in a similar manner to that of Example 1. The profiles ofelements (carbon atoms and oxygen atoms ¹⁸O) within these MIM nonlinearelements were obtained by SIMS in the same manner as in Example 1, withthe results being as shown in FIG. 10. Reference number 1001 in FIG. 10denotes the profile of oxygen atoms (¹⁸O), reference number 1002 denotesthe profile of carbon atoms, reference symbol D1 denotes the region ofthe insulated film, and reference symbol D2 denotes the region of thefirst conductive film. It was determined from FIG. 10 that the relativeintensity of carbon atoms in the insulated film was extremely low atapproximately 0.01 to 0.1, in comparison with the oxygen atoms (¹⁸O).

The relative permittivity of the insulated film and β value wereobtained in a manner similar to that of Example 1, with the resultsbeing as listed in Table 2. Note that reference number 802 in FIG. 8denotes the voltage-current characteristic curve for obtaining the βvalue of Comparative Example 4.

Examples 7 to 12 and Comparative Example 5

The electrolytes listed below were used instead of the electrolyte ofExample 1; otherwise, MIM nonlinear elements were fabricated in asimilar manner to that of Example 1. In other words, an ethylene glycolsolution comprising 9 wt % of diammonium maleate was used as theelectrolyte for Example 7, an ethylene glycol solution comprising 10 wt% of diammonium malonate was used as the electrolyte for Example 8, anethylene glycol solution comprising 8 wt % of diammonium adipate wasused as the electrolyte for Example 9, an ethylene glycol solutioncomprising 10 wt % of ammonium γ-resorcylate was used as the electrolytefor Example 10, an ethylene glycol solution comprising 10 wt % ofammonium benzoate was used as the electrolyte for Example 11, aγ-butyrolactone solution comprising 10 wt % ammonium salicylate was usedas the electrolyte for Example 12, and an aqueous solution comprising 10wt % of ammonium salicylate was used as the electrolyte for ComparativeExample 5.

The relative permittivity of the insulated film and β value of each ofthe thus obtained MIM nonlinear elements were obtained in a mannersimilar to that of Example 1. The results are listed in Table 2.

TABLE 2 Electrolyte Relative Solute Solvent Permittivity β Value E 1Ammonium Ethylene 17.5 6.1 salicylate glycol E 7 Diammonium Ethylene16.0 6.0 maleate glycol E 8 Diammonium Ethylene 18.4 5.6 malonate glycolE 9 Diammonium Ethylene 13.2 5.7 adipate glycol E 10 ammonium γ-Ethylene 18.6 5.4 resorcylate glycol E 11 Ammonium ethylene 15.8 4.3benzoate glycol E 12 Ammonium γ- 20.0 4.2 salicylate butyrolactone CE 4Citric acid Water 27.0 3.8 CE 5 Ammonium Water 25.1 3.9 salicylate E:Example; CE: Comparative Example

As is clear from Table 2, it was determined that the relativepermittivity of the insulated film was small and the β value whichindicates the steepness of the voltage-current characteristic wassufficiently large, in each of these examples of the invention. Incontrast thereto, it was determined that the relative permittivity ofthe insulated film of the comparative examples in which water was usedas the solvent was higher than that of these examples, and also the βvalues thereof were lower.

Example 13

A 200-nm-thick tantalum film was deposited by sputtering on a glasssubstrate, to form a first conductive film. Using an ethylene glycolsolution comprising 10 wt % of tetraethyl-ammonium borate as theelectrolyte, electrolysis at constant current was then performed at acurrent density of 0.1 mA/cm² until the voltage reached 30 V, followedby electrolysis at constant voltage at 30 V for approximately two hours,to subject the tantalum film to anodic oxidation. As a result, atantalum oxide film of a thickness of approximately 50 nm was formed.

Heat treatment at 430° C. was then performed in a nitrogen atmosphere,to stabilize the anodic oxide film (insulated film), then chromium wasdeposited by sputtering to a thickness of 150 nm on that insulated filmto form the second conductive film, completing the MIM nonlinearelement.

The description now turns to the experiment relating to this MIMnonlinear element.

SIMS by cesium ion etching was first performed to obtain the profile ofvarious atoms comprised within this insulated film and first conductivefilm, with the results being as shown in FIG. 11. Thickness from theinsulated film surface towards the first conductive film is plottedalong the horizontal axis in FIG. 11 and numbers of secondary ions areplotted along the vertical axis. Note that the line denoted by referencesymbol a in FIG. 11 is the interface between the insulated film and thefirst conductive film, obtained by using the ¹⁸O profile as reference.

It was determined from FIG. 11 that boron atoms are distributeduniformly in the thickness depth direction through the insulated film.The intensity (number of secondary ions) of boron atoms in the insulatedfilm was determined to be approximately 5×10² to 2.5×10⁴, the intensityof boron atoms in the first conductive film was determined to beapproximately 5×10, with the ratio therebetween (the intensity in theinsulated film with respect to the intensity in the first conductivefilm) being on the order of 10 to 5×10². Note that the intensity ofboron in the first conductive film is indicated by the line denoted byreference symbol b in FIG. 11. It was also determined from FIG. 11 thatcarbon was comprised within the insulated film.

The relative permittivity of an MIM nonlinear element having a 4-μmsquare pattern, fabricated by the above described method, was determinedin a similar manner to that of Example 1 and was found to be 17.1. Thevoltage-current characteristic of this MIM nonlinear element was alsomeasured and the nonlinear coefficient (β value) indicating thesteepness thereof was calculated to be 5.7.

The composition of the electrolyte, the relative permittivity of theinsulated film and β value are listed in Table 3.

When a liquid crystal display panel was fabricated by using MIMnonlinear elements formed by the above described method, a contrast ofat least 100 was obtained over a temperature range of 0° C. to 80° C.,and no display unevenness was discerned.

Comparative Example 6

An electrolyte that was an aqueous solution of 0.1 wt % citric acid wasused instead of the electrolyte of Example 13; otherwise, MIM nonlinearelements were fabricated in a similar manner to that of Example 13. Theprofile of atoms in this MIM nonlinear element were obtained by SIMS ina similar manner to that of Example 13, with the results being as shownin FIG. 12. It was determined from FIG. 12 that additional elementsbelonging to families 3 to 7, other than carbon, were substantially notincluded into the insulated film. The relative permittivity of theinsulated film and β value were obtained in a manner similar to that ofExample 13, with the results being as listed in Table 3.

Examples 14 to 25 and Comparative Example 7

The electrolytes listed below were used instead of the electrolyte ofExample 13; otherwise, MIM nonlinear elements were fabricated in asimilar manner to that of Example 13. In other words, an ethylene glycolsolution comprising 10 wt % of ammonium nitrate was used as theelectrolyte for Example 14, an ethylene glycol solution comprising 10 wt% of tetraethyl-ammonium dihydrogen phosphate was used as theelectrolyte for Example 15, an ethylene glycol solution comprising 10 wt% of tetraethylammonium sulfate was used as the electrolyte for Example16, an ethylene glycol solution comprising 10 wt % oftetraethyl-ammonium vanadate was used as the electrolyte for Example 17,an ethylene glycol solution comprising 10 wt % of tetraethyl-ammoniumchromate was used as the electrolyte for Example 18, an ethylene glycolsolution comprising 10 wt % of tetraethyl-ammonium molybdate was used asthe electrolyte for Example 19, an ethylene glycol solution comprising10 wt % of tetraethyl-ammonium tungstate was used as the electrolyte forExample 20, an ethylene glycol solution comprising 10 wt % oftetraethyl-ammonium perrhenate was used as the electrolyte for Example21, a γ-butyrolactone solution comprising 10 wt % of tetraethyl-ammoniumdihydrogen phosphate was used as the electrolyte for Example 22, aγ-butyrolactone solution comprising 10 wt % of tetraethyl-ammoniumtungstate was used as the electrolyte for Example 23, an ethylene glycolsolution comprising 10 wt % of tetraethyl-ammonium silicate was used asthe electrolyte for Example 24, an ethylene glycol solution comprising10 wt % of tetraethyl-ammonium tungstate and 5.3 wt % of water was usedas the electrolyte for Example 25, and an aqueous solution comprising 10wt % of tetraethyl-ammonium tungstate was used as Comparative Example 7.

The relative permittivity of the insulated film and β value was obtainedfor each of the thus obtained MIM nonlinear elements, in a similarmanner to that of Example 1. The results are listed in Table 3.

TABLE 3 Electrolyte Relative Solute Solvent Permittivity β Value E 13Tetraethyl-ammonium EG 17.1 5.7 borate E 14 Ammonium nitrate EG 20.7 5.6E 15 Tetraethyl-ammonium EG 12.9 6.0 dihydrogen phosphate E 16Tetraethyl-ammonium EG 17.9 5.7 sulfate E 17 Tetraethyl-ammonium EG 18.45.7 vanadate E 18 Tetraethyl-ammonium EG 20.7 5.9 chromate E 19Tetraethyl-ammonium EG 15.6 5.7 molybdate E 20 Tetraethyl-ammonium EG13.8 5.5 tungstate E 21 Tetraethyl-ammonium EG 16.4 5.7 perrhenate E 22Tetraethyl-ammonium GBL 20.8 4.1 dihydrogen phosphate E 23Tetraethyl-ammonium GBL 21.1 4.5 tungstate E 24 Tetraethyl-ammonium EG15.7 5.0 silicate E 25 Tetraethyl-ammonium EG, 22.1 5.5 tungstate waterCE 6 Citric acid Water 27.0 4.3 CE 7 Tetraethyl-ammonium Water 27.0 4.3tungstate E: Example; CE: Comparative Example; EG: ethylene glycol; GBL:γ-butyrolactone

As is clear from Table 3, it was determined that the relativepermittivity of the insulated film was small and the β value indicatingthe steepness of the voltage-current characteristic was sufficientlylarge, in each of these examples of the invention. In contrast thereto,the relative permittivity of the insulated film in each of thecomparative examples was higher than in these examples of the inventionand also the β value was lower.

Example 26

A 200-nm-thick tantalum film was deposited on a glass substrate bysputtering to form the first conductive film. Using an ethylene glycolsolution comprising 10 wt % of ammonium salicylate and 7 wt % of wateras the electrolyte, electrolysis at constant current was then performedat a current density of 0.1 mA/cm² until the voltage reached 30 V,followed by electrolysis at constant voltage at 30 V for approximatelytwo hours, to subject the tantalum film to anodic oxidation. As aresult, a tantalum oxide film of a thickness of approximately 50 nm wasformed.

Heat treatment at 400° C. was then performed in a nitrogen atmosphere tostabilize the anodic oxide film (insulated film), then chromium wasdeposited by sputtering to a thickness of 150 nm on that insulated filmto form the second conductive film, completing the MIM nonlinearelement.

The relative permittivity of the insulated film of an MIM nonlinearelement having a 4-μm square pattern, fabricated by the above describedmethod, was determined in a similar manner to that of Example 1 and wasfound to be 24.3. The voltage-current characteristic of this MIMnonlinear element was also measured and the nonlinear coefficient (βvalue) indicating the steepness thereof was calculated to be 5.5.

To observe changes with time of the voltage-current characteristic ofthe thus obtained MIM nonlinear element, the shift value indicatingchanges with time was obtained and was found to be 1.4%. This shiftvalue is defined as the value I_(S) in the equation below, when arectangular-waveform voltage of a polarity that changes every second wasapplied to the MIM nonlinear element. During this time, the appliedvoltage was set so that the current was 1×10⁻⁷ A for each pixel of theliquid crystal display panel.

I _(S)={(I ₁₀₀ −I ₀)/I ₀}×100(%)

In this equation, I₀ is the initial (1 second) current and I₁₀₀ is thecurrent after 100 seconds.

The water content of the electrolyte, the β value, relative permittivityof the insulated film, shift value, and the pH of the electrolyte arelisted in Table 4.

When a liquid crystal display panel was fabricated by using MIMnonlinear elements formed by the above described method, a contrast ofat least 100 was obtained over a temperature range of 0° C. to 80° C.,and no after-images were discerned.

Examples 27 to 29 and Comparative Examples 8 and 9

Electrolyte including water in the proportions listed below was usedinstead of the electrolyte of Example 26; otherwise, MIM nonlinearelements were fabricated in a similar manner to that of Example 26. Inother words, the proportion of water used for Example 27 was 5 wt %, theproportion of water used for Example 28 was 3 wt %, the proportion ofwater used for Example 29 was 10 wt %, the proportion of water used forComparative Example 8 was 100 wt %, and the proportion of water used forComparative Example 9 was 0.1 wt %.

The β value, relative permittivity of the insulated film, and shiftvalue were obtained for each of the thus obtained MIM nonlinearelements, in a similar manner to Example 26. The results are listed inTable 4, together with the water content and pH of each electrolyte.

Example 30

An electrolyte of ethylene glycol comprising 10 wt % of ammoniumhydrogen phthalate and 5 wt % of water was used instead of theelectrolyte of Example 26; otherwise, MIM nonlinear elements werefabricated in a similar manner to that of Example 26. The β value,relative permittivity of the insulated film, and shift value wereobtained for the thus obtained MIM nonlinear element, in a similarmanner to Example 26. The results are listed in Table 4, together withthe water content and pH of each electrolyte.

TABLE 4 Water Content Relative Shift Value (wt %) β Value Permittivity(%) pH E 26 7 5.5 24.3 1.4 6.4 E 27 5 5.6 21.6 2.3 6.4 E 28 3 5.6 20.826.0 6.4 E 29 10 5.4 19.8 11.9 4.6 E 30 5 5.2 20.0 3.8 6.4 CE 8 Aqueous4.0 27.0 3.0 4.9 solution CE 9 0.1 5.2 18.1 Measurement 6.4 impossibleE: Example; CE: Comparative Example

As is clear from Table 4, it was determined that the relativepermittivity of the insulated film was small the β value indicating, thesteepness of the voltage-current characteristic was sufficiently large,and also the shift value indicating changes with time in thevoltage-current characteristic was sufficiently small, in each of theseexamples of the invention. In contrast thereto, since the water contentof the electrolyte of Comparative Example 8 was too high, the shiftvalue was small, but the relative permittivity increased. The watercontent for Comparative Example 9 was too low, so the β value was largeand also the shift value was so large it could not be measured.

Example 31

MIM nonlinear element of the back-to-back structure shown in FIGS. 6 and7 were used for this example. More specifically, a 150-nm-thick film oftantalum was deposited by sputtering on a glass substrate and waspatterned to form the first conductive film. Using an electrolyte ofethylene glycol comprising 10 wt % of triethyl-methyl-ammonium tungstateand 1.1 wt % of water (pH: 11.1), fixed-current electrolysis was thenperformed at a current density of 0.04 mA/cm² until the voltage reached15 V, to subject the tantalum film to anodic oxidation. As a result, atantalum oxide film of a thickness of approximately 30 nm was formed.

After heat treatment was performed at 400° C. in a nitrogen atmospherefor 30 minutes, the anodic oxide film was cooled in the atmosphere.After the anodic oxide film (insulated film) had been stabilized,chromium was deposited by sputtering to a thickness of 100 nm on thatinsulated film and then was patterned to form the second conductivefilm. The portions of the first conductive film that form the MIMnonlinear element and the portions thereof that would become the signalslines were separated by etching, completing the MIM nonlinear element.

Example 32

An electrolyte comprising water in a proportion of 4.3 wt % was usedinstead of the electrolyte of Example 31; otherwise, MIM nonlinearelements were fabricated in a similar manner to that of Example 31.

Example 33

The voltage during the anodic oxidation was 16 V, the heat treatmentafter the anodic oxidation was at 350° C. for 30 minutes, then steam wasintroduced during the cooling; otherwise MIM nonlinear elements werefabricated in a similar manner to that of Example 31.

Comparative Example 10

An electrolyte comprising water in a proportion of 11.1 wt % was usedinstead of the electrolyte of Example 31; otherwise MIM nonlinearelements were fabricated in a similar manner to that of Example 31.

The description now turns to experiments relating to the MIM nonlinearelements of Examples 31 to 33 and Comparative Example 10.

SIMS

Example 31

SIMS by cesium ion etching was first performed to obtain the profile ofvarious atoms comprised within each insulated film and first conductivefilm, with the results being as shown in FIG. 13. Thickness from theinsulated film surface towards the first conductive film is plottedalong the horizontal axis in FIG. 13 and numbers of secondary ions areplotted along the vertical axis. For this SIMS analysis, the thicknessof the insulated film (oxide film) was set to 45 nm to make the dataeasier to see.

From the spectrum shown in FIG. 13, it was determined that tungsten hadpermeated continuously from the surface of the insulated film formed byanodic oxidation as far as approximately halfway through the thicknessof this insulated film in the MIM nonlinear element of Example 31. Notethat it was also determined that the proportion of the solute within theelectrolyte that permeated into the insulated film is ordinarilyconstant, no matter how the thickness of the insulated film changes.

Examples 31 to 33 and Comparative Example 10

Drift Value

To observe the changes with time of the voltage-current characteristicsof the MIM nonlinear elements of Examples 31 to 33, the drift valuesindicating these changes with time were obtained while a direct currentwas applied, and were found to be 0.33 V, 0.32 V and 0.23 V.

These drift values were obtained by measuring the current-voltage curvetwice for each MIM nonlinear element, defining the resultant voltages asV1 (first measured value) and V2 (second measured value) at a current of1×10⁻¹⁰ A, then obtaining the difference therebetween: ΔV=V2−V1.

The drift value was obtained in a similar manner for Comparative Example10 and was found to be 0.59 V.

Shift Value

To observe the changes with time of the voltage-current characteristicsof the MIM nonlinear elements of Examples 31 to 33, the shift valuesthat indicate changes with time obtained therefor while an alternatingvoltage was applied thereto in a similar manner to that of Example 26,and were found to be −15.3%, −14.5%, and 2.3%. When the shift value wasobtained for Comparative Example 10 in a similar manner, it was −18.4%.

β Value

The voltage-current characteristics of the MIM nonlinear elements ofExamples 31 to 33 were measured and the nonlinear coefficients (βvalues) indicating the steepness thereof were calculated to be 6.0, 6.7,and 7.6. When the β value was obtained for Comparative Example 10 in asimilar manner, it was 6.5.

Relative Permittivity

The relative permittivities of the MIM nonlinear elements of Examples 31to 33 were obtained in a similar manner to that of Example 1, and werefound to be 17.7, 20.5 and 20.5. When the relative permittivity wasobtained for Comparative Example 10 in a similar manner, it was 21.2.

The β value, drift value, shift value, and the relative permittivity ofeach insulated film obtained from the above experimental examples arelisted in Table 5, together with the water content and pH of theelectrolyte. Note that I (4 V) in Table 5 indicates currents obtainedwhen a voltage of 4 V was applied to the MIM nonlinear elements and I(10 V) indicates currents obtained when a voltage of 10 V was appliedthereto.

TABLE 5 Water Shift Relative Content I I β Drift Value Permit- (wt %) pH(4 V) (10 V) Value Value (%) tivity E 31 1.1 11.1 1.3E-11 2.1E-8  6.00.33 −15.3 17.7 (V) E 32 4.3 10.5 4.5E-12 1.4E-8  6.7 0.32 −14.5 20.5(V) E 33 4.0 10.5 6.0E-14 4.7E-10 7.6 0.23 2.3 20.5 (V) CE10 11.1 9.82.1E-12 5.6E-9  6.5 0.59 −18.4 21.2 (V) E: Example; CE: ComparativeExample

As is clear from Table 5, it was determined that the relativepermittivity of the insulated film was small, the β value indicating thesteepness of the voltage-current characteristic was sufficiently large,and also the drift value and shift value both indicating changes withtime in the voltage-current characteristic were sufficiently small, ineach of these examples of the invention. In contrast thereto, since thewater content of the electrolyte of Comparative Example 10 was too high,so the relative permittivity increased, the drift value and shift valuealso increased, and the changes with time of the voltage-currentcharacteristic increased.

When a liquid crystal display panel was fabricated by using the MIMnonlinear elements of Examples 31 to 33, a contrast of at least 100 wasobtained over a temperature range of 0° C. to 80° C., and no displayunevenness was discerned.

Note that it was determined that the electrolytic conductivity of theelectrolyte used in Examples 7 to 33 of this invention were within therange of the electrolytic conductivity of this invention.

What is claimed is:
 1. A method of using an electrolyte for fabricatinga two-terminal nonlinear element comprising a first conductive film, aninsulated film and a second conductive film, wherein said electrolytecomprises an organic solvent, water in a proportion of 1 to 10 wt % anda solute and has electrolytic conductivity being greater than or equalto 1 mS/cm but less than or equal to 100 mS/cm, said first conductivefilm is of tantalum or a tantalum alloy, and said insulated film has arelative permittivity of 10 to 25, comprising the step of: using saidelectrolyte to form said insulated film by anodic oxidation of saidfirst conductive film, said anodic oxidation being at a current densityof 0.01 to 0.1 mA/cm².
 2. The method as defined in claim 1, wherein saidsolute comprises at least one of a carboxylate and a salt of inorganicoxoacid.
 3. The method as defined in claim 2, wherein said carboxylateis at least one salt of carboxylic acids selected from the groupconsisting of aromatic carboxylic acids and aliphatic dicarboxylic acidshaving no hydroxyl groups.
 4. The method as defined in claim 3, whereinsaid carboxylate is at least one type selected from the group consistingof salicylates, resorcylates, benzoate, phthalates, malonates, maleates,and adipates.
 5. The method as defined in claim 2, wherein the centralatom of the oxoacid in said salt of an inorganic oxoacid is an atombelonging to one of families 3 to 7 of the periodic table.
 6. The methodas defined in claim 5, wherein said salt of an inorganic oxoacid is atleast one type selected from the group of nitrates, vanadates,phosphates, chromates, tungstates, molybdates, silicates, perrhenates,borates and sulfates.
 7. The method as defined in claim 1, wherein saidorganic solvent is at least one of ethylene glycol and γ-butyrolactone.8. The method as defined in claim 1, wherein said solute comprises atleast a tungstate.
 9. The method as defined in claim 8, wherein saidtungstate is at least one type of primary, secondary, tertiary, andquartenary ammonium salt.
 10. The method as defined in claim 1, whereinsaid solute comprises at least an aromatic carboxylate.
 11. The methodas defined in claim 10, wherein said aromatic carboxylate is at leastone of salicylate and phthalate.
 12. The method as defined in claim 1,wherein said insulated film comprises carbon atoms through an entirethickness direction of said insulated film.
 13. The method as defined inclaim 12, wherein a relative intensity of said carbon atoms with respectto oxygen atoms (¹⁸O) is 0.2 to 1000 throughout the entire thicknessdirection of said insulated film, as determined by elemental analysisobtained by secondary ion mass spectrometry (SIMS) by irradiation ofcesium primary ions.
 14. The method as defined in claim 1, wherein saidinsulated film comprises at least one element belonging to families 3 to7 of the periodic table distributed through the entire thicknessdirection of said insulated film.
 15. The method as defined in claim 14,wherein a relative intensity of said at least one element in saidinsulated film belonging to families 3 to 7 of the periodic table is atleast 10 times the intensity of said element in said first conductivefilm, as determined by elemental analysis obtained by secondary ion massspectrometry (SIMS) by irradiation of cesium primary ions.